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What should you pay attention to with the autonegotiation function for negotiating the LAN mode in Industrial Ethernet CPs?

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Description:This entry describes what you should pay attention to with regard to the autonegation and autosensing functions in Industrial Ethernet CPs.
The communication processors always try to use the most powerful and fastest contol mode. To understand the following explanaitions, it is necessary to explain some terms about the possible states of the LAN controller when negotiating the LAN operation mode.

Term

Explanation

PHY

Physical level converter that is put in front of the LAN-controler. The PHY passes the telegrams which are on the LAN to the LAN-controler.

Autoswitching

This is a special operation mode of the PHY of the respective module. The PHY is in 10 MBit/sec Half-Duplex mode and is waiting for a corresponding link. In this stat no 100 MBit/sec link will be identified.

Autosensing

Autosensing is the automatic identification of the transfer rate (10/100 MBit/sec).

Autonegotiation

This is the automatic identification/negotiaton of the transfer rate (10/100 MBit/sec) and of the operation mode (Full-Duplex/Half-Duplex).

Table 1: Term explanation

Autonegotiation and Autosensing are supported by all Industrial Ethernet communication processors that support the operation modes 10MBit/sec and 100MBit/sec. The mechanism is used for the automatical identification of the most effective operation mode between two partner devices. Partner devices are (usually) a communication processor or a network component.

The following operation modes can be negotiated:

Baud rate

Operation mode

10 Megabit

Half-Duplex

10 Megabit

Full-Duplex

100 Megabit

Half-Duplex

100 Megabit

Full-Duplex

Table 2: Summary of the possible baudrates and operation modes.

Meaning of the operation modes

Full-Duplex means that a bidirectional data exchange is possible. On the transmission network each of both users can send independently from each other.

In Half-Duplex operation a bidirectional data exchange is possible as well. On the transmission network, however, always only one data transport in one direction at one time between two components is possible. A data transfer between two components can not take place at the same time, it must be done by turns.

Purpose of this methodThis is aimed to get the highest possible baudrate with the best possible operation mode - that is a baudrate of 100 MBit of the control mode fullduplex.

Two different kinds of approach have to be distinguished for negotiating the LAN operating mode. Since August 2001, the mechanism has been changed to guarantee an automatic coupling to almost all partner stations. In the following, the different versions V1 and V2 are described. Which modules or firmware versions belong to which group is described below.

Hint:Autonegotiation is a 100 MBit/sek attribute, which is not available for modules with only one AUI interface, or an ITP interface with 10MBit/sec or a fix preselection in the hardware configuration.

Description of the negotiation procedure V1:After starting the module the communication processor executes the autonegotiation. If it is supported by the partner, the highest possible operating mode will be negotiated. The active communication processor will recommend 100MBit/sec full-duplex, and if required it will switch to the supported operating mode of the parter unit.

If it does not receive an answer because the partner does not support autonegotiation, the mode will switch to "Autoswitching". This would be the same as 10 MBit/sec half-duplex. It is the default control mode of the communication processor.

All units which only support 10MBit/sec half-duplex will not answer to the autonegotiation pulse. A communication is after this is still possible because the partner station has been set to the default.

If no partner was available after the start-up, the default setting is still active. If a partner responds later, a 10MBit/sec link will be created at first. After this the communication processor initiates another autonegotiation activity, because the partner might support more than 10MBit/sec.
Both partners can now agree on a 100MBit/sec full-duplex, it both support it.
If the communication processor receives no answer to a autonegotiation, it will remain on the 10 MBit/sec half-duplex.

Hint:If a partner just supports 100MBit/sec full-duplex, but also autonegotiation, two cases have to be distinguished. Such a unit is for excample the OMC (optical media converter):

Are both partners connected while starting-up, the first autonegotiation attempt will lead to a negotiation of the communication processor with 100 MBit/sec full-duplex. During the autonegotiation, a 100MBit/sec link will be identified and afterwards the control mode full-duplex.

If the 100 MBit/sec component is connected later, the communication processor is already in the autoswitching mode. This is the result of the first unsuccessful autonegotiation, and now a 10 MBit/sec link is absolutely necessary. Since the partner station (OMC) does not support 10 MBit/sec, a connection can never be established.

Moduls which support the autonegotiation V1:

Module

Moduletype

Firmwareversion

6GK7 343-1EX10-0XE0

CP 343-1

to version V1.0.3

6GK7 443-1EX10-0XE0

CP 443-1

to version V1.1.0

6GK7 443-1EX11-0XE0

CP 443-1

to version V1.1.0

6GK7 343-1GX00-0XE0

CP 343-1 IT

to version V1.0.1

6GK7 443-1GX10-0XE0

CP 443-1 IT

to version V1.1.0

6GK7 443-1GX11-0XE0

CP 443-1 IT

to version V1.1.0

Table 3: Modules which support Autonegotiation V1

Description of the negotiation procedure V2:

The following changes have been implemented because of the OMC properties:
The communication processor switches cyclycally between autoswitching and autonegotiation (after start up and disappearance of a link). This way, a corresponding component can be coupled under any circumstances. Therefore, all cases described above can be controlled.
Feature:
While switching, the FAST-LED will blink slowly.
The communication processor finally changes into the status AUI (corresponding to 10 MBit/sec half-duplex) if it receives telgrams via the AUI cable (activitiy control). Autoswitching is started at the same time to identify a possible 10 MBit link of an interim placed twisted pair-cable.
If such a link was identified, autonegotiation will be started again to negotiate the highest possible operational mode between both partners.

Modules which support autonegotiation version V2:All the current Industrial Ethernet CPs, except for those listed in Table 03, support autonegotiation version V2.

Hint:
If the operating mode "100 MBit/sec full-duplex" has been negotiated or set with project planning, a sporadical loss of telegrams might occur. This affects only high demand cases where the CP itself needs to process the larger part of telegram communication.
This effect will be amplified if the CP has a lot of partner stations which are able to send and receive asynchronously.

The circumstance described apply to the following Industrial Ehternet Communication processors:

Modules

Moduletype

Firmwareversion

6GK7 443-1EX10-0XE0

CP 443-1

to version V1.1.0

6GK7 443-1EX11-0XE0

CP 443-1

to version V1.1.0

6GK7 443-1GX10-0XE0

CP 443-1 IT

to version V1.1.0

6GK7 443-1GX11-0XE0

CP 443-1 IT

to version V1.1.0

Table 4: Modules with restrictions in full-duplex operation

The effect appears in temporary "dropouts" on a corresponding connection. Due to the telegram loss the partner station is waiting on an acknowledge on transmission level. This will not happen. After about one second the partner station repeats the last telegram sent and the communication continues. Because of this behaviour the performance decreases. Neither the connection will be closed nor will the telegram be lost, since the superposed transport layer will provide for a repeat of the telegrams.

Remedy:
The communication partners (CP and network components) have to be set fixed to an operating mode unequal to "100 Mbit/sec full duplex", that means no autonegotiation. It is important here that both partners are set fixed, because otherwise different operating modes might occur in the two communication partners.
This behavior does not occur in the following firmware versions of the communication processors listed in Table 04 and in the current Industrial Ethernet CPs. Then in none of the operating modes will messages be lost.

Keywords:Network, network protocol, data loss

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